AVHRR-Pathfinder

Mission Specification

Multichannel sea surface temperature (SST) products have been constructed operationally from the five and six channel Advanced Very High Resolution Radiometer (AVHRR) by NOAA's National Environmental Satellite, Data, and Information Service (NESDIS) since late 1981.

This Pathfinder SST time series represents the longest continual global ocean physical measurement from space. Development of the Pathfinder dataset (including all Version 5.0 and above data sets) is sponsored by the NOAA National Oceanographic Data Center (NODC) in collaboration with the University of Miami Rosensteil School of Marine and Atmospheric Science (RSMAS) while distribution is a collaborative effort between the NASA Physical Oceanography Distributed Active Archive Center (PO.DAAC) and the NODC. From a historical perspective, the Pathfinder program was originally initiated in the 1990s as a joint NOAA/NASA research activity for reprocessing of satellite based data sets including SST.

This research quality data consists of daily, 5 day, 8 day, monthly and yearly level 3 products on a 4km global grid. As of January 2011 it includes data from August 24, 1985 through December 31, 2009. Pathfinder AVHRR data comes from NOAA -9 (1/4/1985 – 11/07/88 & 9/14/94 – 01/21/95), NOAA – 11 (11/8/88 – 9/13/94), NOAA-14 (1/22/95 – 10/11/2000), NOAA-16 (10/12/00 -12/31/02), NOAA-17 (01/01/03 – 06/04/05), and NOAA-18 (06/05/05 – 12/31/09) satellites.
In 2009 NODC released Pathfinder version 5.1 from October of 1981 through January of 1985. This previously un-released data was from the NOAA -7 satellite. Version 5.1 uses higher resolution reference data than 5.0 for the purpose of achieving better resolution in the coastal and high gradient SST regions.
In September of 2011 the Pathfinder Version 5.2 (PFV5.2) was released. Several significant changes and improvements occurred with the release of PFV52. The following description may be found at the NOAA/NODC web site (http://www.nodc.noaa.gov/SatelliteData/pathfinder4km/).

"PFV52" was computed using an entirely modernized system, based on SeaDAS and incorporating several key changes over the older Pathfinder V5.0 and V5.1 datasets. PFV52 is viewed as a significant step forward in preparation for the future Version 6 (PFV6) data set (no release has yet been established for PFV6). Changes in PFV52 include the use of an entirely new land mask, a modified grid, and the inclusion of sea ice and wind speed ancillary data to support the use of the SST data. Importantly, the new PFV52 data are provided in netCDF-4 (classic model, with internal compression and chunking) and are nearly 100% compliant with the GHRSST Data Specification Version 2.0 (GDS2.0,http://data.nodc.noaa.gov/pathfinder/Version5.2/GDS_TechSpecs_v2.0.pdf) for L3C products. These data deviate from that standard only in that the Single Sensor Error Statistics (SSES) which include the sses_bias, sses_standard_deviation are empty. Additionally the sst_dtime variable is empty. Data for 1981-2010 are available. All of the PFV52 data are available through NODC's ftp, http, OPeNDAP, and THREDDS access systems.” More information on data gaps, processing may be found at: http://www.nodc.noaa.gov/SatelliteData/pathfinder4km/ .

All the orbits are sun synchronous and have an ascending node of between 13:30 and 14:20 local time with the exception of NOAA-17 that has a daytime descending node of approximately 10:00.

AVHRR Instrument

The AVHRR/3 is a 6 channel imaging system in which a small field of view (1.3 milliradians by 1.3 milliradians) is scanned across the earth from one horizon to the other by continuous 360 degree rotation of a flat scanning mirror.

The orientation of the scan lines are perpendicular to the spacecraft orbit track and the speed of rotation of the scan mirror is selected so that adjacent scan lines are contiguous at the subsatellite (nadir) position. A total of 2048 samples will be obtained per channel per Earth scan, which will span an angle of 55.4 degrees from the nadir (subpoint view). All 6 spectral channels of the AVHRR/3 are registered so that they all measure energy from the same spot on the earth at the same time. All 6 channels are also calibrated so that the signal amplitude in each channel is a measure of the scene radiance. The radiometers are designed to operate for a period of 3 years.

The Polar Operational Environmental Satellites (POES) system offers the advantage of daily global coverage, with morning and afternoon orbits that deliver global data. Data includes surface temperature, cloud cover, storm location and heat balance in the earth's atmosphere. The NOAA-x series have been performing earth observations since 1970.
The AVHRR/2 is a 5 channel imaging system in which a small field of view (1.3 milliradians by 1.3 milliradians) is scanned across the earth from one horizon to the other by continuous 360 degree rotation of a flat scanning mirror. The orientation of the scan lines are perpendicular to the spacecraft orbit track and the speed of rotation of the scan mirror is selected so that adjacent scan lines are contiguous at the subsatellite (nadir) position. A total of 2048 samples will be obtained per channel per Earth scan, which will span an angle of 55.4 degrees from the nadir (subpoint view). All 5 spectral channels of the AVHRR/3 are registered so that they all measure energy from the same spot on the earth at the same time. All 5 channels are also calibrated so that the signal amplitude in each channel is a measure of the scene radiance. The radiometers are designed to operate for a period of 3 years.

PO.DAAC currently provides complete time series for 3 currently active AVHRR SST data sets. These include 1 level 3 and 2 level 2 satellite SST sets.

The level 3 data is Pathfinder version 5. This provides research quality data from 1981 through the near present. It is longest running global SST data set available anywhere. This 4km resolution data is processed every view months. As of June 2009 data was available through May of 2009. Pathfinder data is produced by NOAA's National Oceanographic Data Center (NODC).

The two level 2 data sets are Multi-Channel Sea Surface Temperature (MCSST) products provided by the Naval Oceanographic Office (NAVOCEANO) in near real time. One product focuses within 1000km of all global coastlines (with the exception of some small islands) with an additional focus on the Western Pacific. This data has a resolution of 2.2km. The other near real time NAVOCEANO product is global, with a resolution of 9km.

The other 2 active products derived using AVHRR SST are the National Centers for Environmental Prediction's (NCEP) optimally interpolated and historically reconstructed data sets. These two sets combine AVHRR as well as in-situ data

About NOAA series:

The NOAA series of satellites have been providing global coverage of the Earth and Ocean's for decades. They are part of the Polar Operational Environmental Satellite's (POES) network. POES uses two primary satellites, an AM and a PM placed in sun synchronous orbits such that they provide global coverage 4 times per day.

As of August 2009 POES primary satellites are equipped with at least the following sensors:

AVHRR/3: Advanced Very High Resolution Radiometer

HIRS/4: High resolution Infrared Radiation Sounder

AMSU-A1: Advanced Microwave Sounding Unit I

AMSU-A2: Advanced Microwave Sounding Unit 2

MHS: Microwave Humidity Sounder

SARR: Search and Rescue Repeater

SARP-3: Search and Rescue Processor

SEM: Space Environment Monitor.

These instruments deliver important ocean, atmosphere, and space radiation data as well as aiding ships and aircraft in emergency situations. The AVHRR instrument has been providing continuous record of sea surface temperature since 1980. This research quality data set named Pathfinder can be found here.(http://nssdc.gsfc.nasa.gov/planetary/mesur.html).

New NOAA series satellites are launched in advance so that when an operational satellite begins to fail another satellite is in orbit ready to replace it. The orbits are sun synchronous meaning the orbits are fixed in relation to the sun. This causes the satellites to cross the equator at the same local times each orbit. The PM satellites ascend across the equator at between 13:30 and 15:00 and while the AM satellites descend at ~7:30 local time. PM satellites descend at between 1:30 and 3:00 while the AM satellites ascend across the equator at ~1900.

All NOAA satellites since the NOAA-6 have been in a sun synchronous orbit so that they orbit the earth slightly more than 14 times per day. With the ~3000km swath of most of their sensors every place on the globe is scanned at least once every 6 hours.